1. Field of the Invention
The present invention relates to an image pickup apparatus for imaging an object.
2. Description of the Related Art
A first known art, disclosed in U.S. Pat. No. 3,971,065, is the most general known art used for taking a color image signal output from an image pickup device. This official gazette discloses a configuration in which an absorptive material, which selectively absorbs light in a specific wavelength range, is disposed on individual pixels forming an image pickup device so that a photoelectric signal corresponding to a color signal is output. Further, in this known art, this absorptive material is arranged in a two-dimensional mosaic pattern so that a color image signal is generated.
On the other hand, various ideas for a configuration for outputting photoelectric signals corresponding to a plurality of color signals from a pixel are suggested.
Japanese Patent Laid-Open No. 7-074340 discloses an image pickup device including a plurality of photoelectric conversion layers which are disposed at different depths from the surface of a semiconductor substrate and which are electrically separated each other, the image pickup device outputting photoelectric conversion signals corresponding to different spectral sensitivities from a pixel. In this second known art, a plurality of photosensitive layers are formed at different depths from the surface of the semiconductor substrate such that the layers are insulated by a p-type impurity region. By adequately setting the positions of these photosensitive layers, a desired color signal can be output.
Also, a third known art, U.S. Pat. No. 5,965,875, discloses a two-dimensional image sensor of a three-layer photodiode structure, in which a photodiode is formed in a triple-well structure. In this third known art, an n-type layer, a p-type layer, and an n-type layer are formed in this order on the surface of a p-type silicon substrate so that a pn-junction diode forms a three-layered photoelectric conversion region in the depth direction of silicon.
In the first known art, the transmittance in a specific wavelength region in the visible wavelength region can be increased by selectively absorbing light in the specific wavelength region. However, a suitable material which absorbs light in the infrared wavelength region other than the visible wavelength region at the same time is difficult to find. Thus, an optical filter having a function of blocking light in the infrared wavelength region has been generally used in combination. As the optical filter, a dichroic mirror including a dielectric multilayer film evaporated on a transparent substrate or a filter in which the spectral characteristic is adjusted by changing material composition as disclosed in Japanese Patent Laid-Open No. 10-101370 has been used. Also, in the second or third known art, a photoelectric signal is output for light in a wide wavelength region from the visible wavelength region to the near infrared wavelength region according to the photoelectric conversion characteristic of the semiconductor. Thus, an optical filter for blocking light in the infrared wavelength region has been needed.
In the first known art, an absorptive material, having a spectral sensitivity for allowing light in a desired wavelength region to pass through corresponding to a color signal, is disposed for each pixel, and a plurality of types of absorptive material are disposed in a mosaic pattern. Accordingly, a two-dimensional image for each color is formed so that a color image is generated. At this time, each color signal is discrete according to the arrangement of the absorptive material, and thus interpolating must be performed by using the color signal of an adjoining pixel. As a result, a finally generated image has an advantage in that a desired spectral sensitivity of each color signal can be obtained. However, the contrast of color signals decreases and a false-color signal is disadvantageously generated.
In the second and third known arts, the configuration of an image pickup device for outputting a plurality of color signals from a pixel is disclosed in order to solve the problems of the first known art. In these known arts, the problems of the contrast of color signals and generation of a false-color signal can be solved, but it is difficult to obtain a desired spectral characteristic of each color. As a result, it becomes difficult to generate a color signal of a taken image similar to the color signal of an object which has been observed by a user, when a color image is generated.